The capacity of an optical fiber to transmit digital information in the form of light pulses far exceeds the capacity of electrical conductors to carry information in the form of electrical pulses. Consequently, numerous information transmission systems in which information is transmitted as a sequence of light pulses in an optical fiber have been constructed or proposed.
The information which is normally communicated in such transmission systems is initially in the form of electrical signals. These signals are typically digital data such as that stored in a computer or generated by modern telecommunications equipment. The rate at which digital data is generated is usually significantly lower than the transmission capacity of an optical fiber. As a result, the data is typically accumulated in a register or other storage system until sufficient data to fill a data packet of a predetermined size has been accumulated. The contents of this data store are then read out to the optical fiber.
At the other end of the optical fiber, the information is typically shifted into a register. The data in this register is then transmitted to the intended receiving device at a speed consistent with the input bandwidth of the receiving device.
Even with currently available electrical circuitry, the rate at which information can be transmitted is significantly lower than that of the devices that interface to the data to the fiber. Consequently, modern transmission systems use a single optical fiber to service a plurality of devices. Such systems utilize time-domain multiplexing. For example, each device accumulates the data it wishes to send. The accumulated data is then inserted into a predetermined time slot in the optical data stream. Similarly, each device includes a circuit which monitors the optical data stream for packets in a predetermined time slot intended for that device. The packets are typically read into buffer registers whose contents are transferred to the device.
The number of devices that may share a single optical fiber depends on the rate at which data can be inserted into the optical transmission stream. In a typical shared transmission system, each device is interfaced to the optical fiber by an interface which is inserted into the fiber. The interface includes a light detector and a laser diode. The interface intercepts each data packet in the segment of the fiber connected to the light detector. Those packets that are not intended for the device connected to the interface are reproduced in the segment of the fiber connected to the laser diode by pulsing the diode. When the device connected to the interface wishes to send a packet, the interface waits for the appropriate time slot and then inserts light pulses into the segment of the fiber connected to the laser diode.
From the above discussion, it will be apparent that the rate at which information can be input to the optical fiber is dependent on the rate at which electrical signals can be generated in response to the light pulses detected in the optical fiber. The rate is also limited by the rate at which the laser diode can be turned on and off. That is, the effective bandwidth of the optical fiber is determined by the rate at which electrical signals can be turned on and off in conventional electrical circuitry. As pointed out above, this rate is significantly lower than that specified by the optical bandwidth of the fiber. Hence, it would be advantageous to provide an interface circuit whose switching speed is not limited by the rate at which electrical signals can be turned on and off.
Broadly, it is an object of the present invention to provide an improved electrical-optical interface unit.
It is a further object of the present invention to provide an electrical-optical interface unit that can switch data into optical transmission paths at a rate which is not limited by the rate at which electrical signals can be turned on and off.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.